1. Field of the Invention
The present invention generally relates to a light transmitter, in particular, to a transmitter and an automatic power control (APC) circuit thereof.
2. Description of Related Art
With the increase of people's demands and the diversification of consumptive electronic products, some short-distance transmission electronic devices, such as infrared ray remote controls, toy car remote controls, and access control systems, have gradually become indispensable tools in people's life. Besides, relative applications can be found in digital storage devices, for example, laser diodes applied in optical pickup heads of digital versatile disk (DVD) drives.
The characteristic curve of the above laser diode may change with the rising temperature during the usage. As shown in FIG. 1, the first characteristic curve 101 is the characteristic curve of the laser diode when working at a normal temperature, and the second characteristic curve 102 is the characteristic curve after the temperature rises. In FIG. 1, the horizontal axis is the current input, and the longitudinal axis is the power output. The turning points of the first and second curves represent a turn-on point of the laser diode at the specific working temperature. Referring to FIG. 1, a first bias current IB1 is provided at a normal temperature, and a corresponding output power is obtained as a first output power P1. Then, a first modulation current IM1 is provided and a corresponding first swing output power 103 may be obtained. At this time, if the temperature rises, the characteristic curve of the laser diode changes into the second curve 102, so the originally provided first bias current IB1 is unable to turn on the laser diode. Even if the first bias current IB1 is raised to a second bias current IB2 to turn on the laser transistor, in order to achieve the swing output power the same as that obtained at the normal temperature, a second modulation current IM2 greater than the first modulation current IM1 must be provided.
In view of the above, a fixed driving current is unable to control the power output of the laser diode, so a conventional automatic power control (APC) circuit is provided. FIG. 2 is a conventional double-loop APC circuit, in which a laser diode module 210 includes a group of laser diodes LD for generating a light signal, and a photo diode serving as a photo detector PD. A cathode of the laser diodes LD is coupled to a bias current source 203 through a ferrite bead 204. A driver 201 receives a pair of differential input signals VIN1 and VIN2, and outputs a modulation signal to the cathode of the laser diodes LD. The amplitude of the modulation signal is determined by a modulation current source 202. The operating mode of the power control circuit is described as follows. The photo detector PD detects a current of the laser diodes LD, and the current is converted into a voltage signal by a trans-impedance amplifier (TIA) 205. Then, the converted voltage signal is transmitted to a valley detector 208 and a peak detector 209 respectively, so as to detect a peak value and a valley value of the current, and transmit the peak value and the valley value respectively to comparators 206, 207. The comparator 206 controls the bias current source 203 based on a result of comparison between the valley value and a first reference voltage VREFB1. The comparator 207 controls the modulation current source 202 based on a result of comparison between the peak value and a second reference voltage VREFM1. Thus, the output power may be automatically controlled by controlling the above two current sources. The first reference voltage VREFB1 and the second reference voltage VREFM1 are preset fixed voltages.
In the aforementioned conventional art, a photo detector PD is used as a detecting element. Thus, if the speed of light communication is faster, a high-speed photo detector PD is needed, which may greatly increase the cost.